Re: [Qemu-devel] [PATCH] mlock: fix bug when mlockall called before mbind

[zhanghailiang]

On 2014/8/14 15:15, Hu Tao wrote:
> On Thu, Aug 14, 2014 at 02:31:41PM +0800, zhanghailiang wrote:
> > On 2014/8/13 19:50, Michael S. Tsirkin wrote:
> > > On Wed, Aug 13, 2014 at 07:21:57PM +0800, zhanghailiang wrote:
> > > > If we configure qemu with realtime-mlock-on and memory-node-bind at the same 
> > > > time,
> > > > Qemu will fail to start, and mbind() fails with message "Input/output error".
> > > >
> > > > >  From man page:
> > > > int mbind(void *addr, unsigned long len, int mode,
> > > >                   unsigned long *nodemask, unsigned long maxnode,
> > > >                   unsigned flags);
> > > > The *MPOL_BIND* mode specifies a strict policy that restricts memory allocation
> > > > to the nodes specified in nodemask.
> > > > If *MPOL_MF_STRICT* is passed in flags and policy is not MPOL_DEFAULT(In qemu
> > > > here is MPOL_BIND), then the call will fail with the error EIO if the existing
> > > > pages in  the memory range don't follow the policy.
> > > >
> > > > The memory locked ahead by mlockall can not guarantee to follow the policy 
> > > > above,
> > > > And if that happens, it will result in an EIO error.
> > > >
> > > > So we should call mlock after mbind, here we adjust the place where called 
> > > > mlock,
> > > > Move it to function pc_memory_init.
> > > >
> > > > Signed-off-by: xiexiangyou<address@hidden>
> > > > Signed-off-by: zhanghailiang<address@hidden>
> > >
> > > OK but won't this still fail in case of memory hotplug?
> > > We set MCL_FUTURE so the same will apply?
>
> It has already been set.
>
> > > Maybe it's enough to set MPOL_MF_MOVE?
> > > Does the following work for you?
> >
> > Hi Michael,
> >
> > I have tested memory hotplug, use virsh command like
> > 'virsh setmem redhat-6.4 6388608 --config --live', it is OK, and
> > it will not call mbind when do such memory hotplug.
> > but i don't know if there is command like 'memory-node hotplug' ?
>
> Using qemu monitor command:
> object_add memory-backend-ram,id=ram1,size=128M,host-nodes=0,policy=bind

Hi Hu Tao,

I have tested it use the above command, and yes, it failed.
And if i remove the *MCL_FUTURE* flag of mlockall(), it will work fine.

*MCL_FUTURE*  Lock  all  pages  which  will become mapped into the
address space of the process in the future.(From man page)

So i think we could remove this flag, and call mlockall(MCL_CURRENT)
every time when we do the above 'memory object add'.

> > MPOL_MF_MOVE can work, it is more simple, but it is not perfect. It
> > consumes more time to *move the memory*(i guess will reconstruct
> > pages and copy memory)which has been locked by mlockall. The result
> > is VM will start slower than the above scenario.
>
> I think your patch makes sense but it doesn't work for hotplugged
> memory. We need MPOL_MF_MOVE, too.

Thank you very much, before send another patch, I will look into the
qemu process of hotplug memory, and try to solve this fault.:)

> > BTW, i think the follow process is clearer and more logical:
> > Allocate memory--->Set memory policy--->Lock memory.
> >
> > So what's your opinion? Thanks very much.
> >
> >
> > > -->
> > >
> > > hostmem: set MPOL_MF_MOVE
> > >
> > > When memory is allocated on a wrong node, MPOL_MF_STRICT
> > > doesn't move it - it just fails the allocation.
> > > A simple way to reproduce the failure is with mlock=on
> > > realtime feature.
> > >
> > > The code comment actually says: "ensure policy won't be ignored"
> > > so setting MPOL_MF_MOVE seems like a better way to do this.
> > >
> > > Signed-off-by: Michael S. Tsirkin<address@hidden>
> > >
> > > ---
> > >
> > > diff --git a/backends/hostmem.c b/backends/hostmem.c
> > > index ca10c51..a9905c0 100644
> > > --- a/backends/hostmem.c
> > > +++ b/backends/hostmem.c
> > > @@ -304,7 +304,7 @@ host_memory_backend_memory_complete(UserCreatable *uc, 
> > > Error **errp)
> > >           /* ensure policy won't be ignored in case memory is preallocated
> > >            * before mbind(). note: MPOL_MF_STRICT is ignored on hugepages so
> > >            * this doesn't catch hugepage case. */
> > > -        unsigned flags = MPOL_MF_STRICT;
> > > +        unsigned flags = MPOL_MF_STRICT | MPOL_MF_MOVE;
> > >
> > >           /* check for invalid host-nodes and policies and give more verbose
> > >            * error messages than mbind(). */
>
>
> .